Method and means for detecting incipient capacitor failure

ABSTRACT

Incipient failure of a capacitor operating as a part of an induction furnace circuit is detected by monitoring the capacitor to determine the presence of high-frequency signals generated by the capacitor as its dielectric material deteriorates. The lower limit of the frequencies of such signals is in the range from 50 to 200 kHz. The sensing apparatus for a 3 kHz induction heating circuit includes detector means for converting the generated signals exceeding about 150 kHz into a control signal whose level exceeds a predetermined value when the generated signals are representative of incipient deterioration of the dielectric. Indicator means responsive to the control signal is operated when the latter signal exceeds said predetermined value, and may disconnect power from the capacitor.

United States Patent Boehm et al.

[15] 3,657,602 [451 Apr. 18, 1972 [72] Inventors: Paul C. Boehm;Theodore R. Kennedy,

both of Willingboro, NJ. [73] Assignee: Inductotherm Corporation,Rancocas, NJ.

22 Filed: Apr. 5, 1971 [21] App]. No.: 131,015

[52] U.S.Cl. ..3l7/12 R,317/53,324/54, 324/60, 340/253 X [51] Int. Cl...I-I02h 7/16 [58] FieldofSearch ..317/12 A, 12 B, 12 R,53; 324/54, 60R, 60 C; 340/253 Y, 248; 307/129 [56] References Cited UNITED STATESPATENTS 2,617,859 11/1952 Kraft ..324/60 R CON TROL FOR POWER INPUT TOPOWER INPUT TO TA NK C IRC U/ T 3,015,774 1/1962 Eigen ..324/542,363,898 11/1944 Partington Primary Examiner.l. D. Miller AssistantExaminer-Harvey Fendelman Attorney-Seidel, Gonda & Goldhammer [5 7]ABSTRACT Incipient failure of a capacitor operating as a part of aninduction furnace circuit is detected by monitoring the capacitor todetermine the presence of high-frequency signals generated by thecapacitor as its dielectric material deteriorates. The lower limit ofthe frequencies of such signals is in the range from 50 to 200 kHz. Thesensing apparatus for a 3 kHz induction heating circuit includesdetector means for converting the generated signals exceeding about 150kHz into a control signal whose level exceeds a predetermined value whenthe generated signals are representative of incipient deterioration ofthe dielectric. Indicator means responsive to the control signal isoperated when the latter signal exceeds said predetermined value, andmay disconnect power from the capacitor.

10 Claims, 1 Drawing Figure TANK CR7:

PATENTEDAPR m 1972 INVENTQRS PAUL C. EOE/1M THEODORE R. KENNEDY 8y wwmmm ATTORNEYS METHOD AND MEANS FOR DETECTING INCIPIENT CAPACITOR FAILUREDETAILED DESCRIPTION This invention is concerned with a method and meansfor determining incipient failure of a capacitor operating as part of aninduction heating or melting circuit.

Banks of capacitors are conventionally used as a part of a tank circuitof an induction heating and melting furnace. Such capacitors have afinite life; and are subject to failure during furnace operation. It isessential, in view of the size of these capacitors and the possibledanger to life and property in the event of failure under load to detectincipient failure and to 'shut down the power input to the tank circuitbefore failure actually occurs.

The causes of capacitor failure are built into a capacitor by reason ofits'inherent operating characteristics. An A.C. load impressedon acapacitor is always accompanied by power losses due to dielectricabsorption, the flow of leakage current, plate resistance, etc. All ofthese power losses are converted to heat which raises the temperature ofthe capacitor. As 'the' temperature rises, the resistance of theinsulating material in the capacitor decreases, thus increasing theleakage current. Such current may be concentrated in a few paths of lowcross section locally heating the dielectric and further reducing itslocal resistance and increasing the leakage current. As the dielectricof the capacitor is heated, rapid deterioration is likely to occur,particularly if moisture is present. Ultimately, breakdown resultsfollowed by flash-over and corona discharge.

Capacitor failure can occur in two modes. In the initial mode, a highimpedance fault develops permitting internal arcing to occur, sometimeson each cycle of operation. Such arcing is manifested by a rumblingsound. If permitted to remain on the line, the container of thecapacitor is subject to increasing internal pressure as gases generatedby the arcing build up. The least that may occur under this condition isa rupture of the container with resultant spillage of insulating oil. Insome instances, an explosion can take place. So far as is known, notechnique is presently available to detect this mode of failure exceptto listen for the rumbling sound.

In another mode of failure, an internal short circuit is developed,sometimes as a result of arcing. This second mode of failure cansometimes be detected successfully when the increase in current causedby the short circuit trips the overload circuit on the main powersupply. However, reliance on this approach to provide protection is notsatisfactory when the incremental increase in power required by theshorted capacitor is within the range of operation of the overload tripon the main power supply, or when the response of the overload trip isslower than the build-up of heat and pressure in the con- .-tainer ofthe capacitor. Furthermore, a faulty capacitor may never short and wouldremain in operation undetected.

It is therefore a primary object of the present invention to provide amethod and means for detecting incipient capacitor failure withoutrelying upon increases in the load current.

It is a further object of the present invention to provide a method andmeans for detecting incipient capacitor failure of only one of a bank ofcapacitors connected in parallel.

Briefly, the invention is based on a discovery that a capacitor in theinitial stages of deterioration will generate highfrequency signals asdielectric breakdown occurs. The lower limit of such signals is in thefrequency range 50 to 200 KHZ. By monitoring the capacitors for thepresence of such high frequency signals, and by converting them to acontrol signal whose level will exceed a predetermined value when thedeterioration reaches a significant level, it is possible to operate anindicator means which will signal the deteriorating condition and/oroperate a control circuit which will remove power from the capacitorbank.

The features of this invention for which protection is sought arepointed out with particularity in the appended claims. The inventionitself, however, both as to its organization and method of organization,together-with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawing, wherein like parts in the single FIGUREof the drawing are identified by the same reference character, andwherein the single FIGURE is an electrical schematic showing means fordetecting incipient capacitor failure.

Referring now to the drawing, reference numeral 10 designates apparatussuitable for protecting tank circuit 11 which is representative of atank circuit used as part of an induction heating or melting furnace(not shown). Apparatus 10 comprises first circuit means 12, secondcircuit means 13, operable indicator means 14, reset circuit 15, andpower supply 16.

For induction melting furnaces below about Kw, the resonant frequency ofthe tank circuit is of the order of magnitude of 3 K112; and it is inthis environment that the circuit shown in the drawing operates. Forillustrative purposes, the tank circuit is shown as a parallelcombination of inductor L1, and capacitors Cl and C2. The tank circuitis supplied with operating power by a suitable oscillator circuit andswitch (now shown), whose operation is under the influence of controlmeans 17. As long as switch S1 is closed, control means 17 is operativeand will effect the application of power to tank circuit 11. When switchS1 is opened, control means 17 will be inoperative and power will beremoved from tank circuit 11.

In tank circuit 11, capacitors Cl and C2 represent the capacitors whoseperformance is being monitored by apparatus 10. First circuit means 12of apparatus 10 is connected directly across capacitors Cl and C2 of thetank circuit by terminals 18 and 19. Circuit means 12 compriseshigh-pass filter 20, isolation transformer 21 and detector circuit 22.Filter 20 includes capacitor C3 in series with primary winding 23 ofisolation transformer 21. The values of these components, when theresonant frequency of the tank circuit is about 3 KI-lz, are selectedsuch that they will operate as a highpass filter circuit attenuatingsignals appearing at terminals 18 and 19 in the frequency range belowabout KHZ by about 60 db. Filter 20 thus will effect the transmission ofsignals greater than about 150 kHz, and apply such signals to detectorcircuit 22. The detector circuit, which is associated with secondary 24of transformer 21, includes diode D1 and capacitor C4, and will producea rectified control signal whose level is functionally related tosignals passed by filter 20.

Second circuit means 13 comprises transistors Q1 and Q2 together withtheir associated bias circuitry. In the absence of a control signal atnode 25 at the output of the detector circuit, both transistors Q1 andQ2 will be in a non-conducting state. Transistor O1 is cut off becauseits base-emitter junction will be reverse-biased by reason of the flowof current from ground terminal 26 of the power supply and ground bus 35through primary 24 of transformer 21, diode D1, and resistors R1 and R2to the 20 volt terminal 27 of power supply 16. By a proper selection ofcomponents, it is possible to maintain node 28 at the base of transistorQ1 at a voltage of about -2 volts with respect to the voltage of theemitter of this transistor. Being cut off, no collector current can flowin this transistor with the result that no base current will flowthrough transistor Q2. As a consequence, this transistor will also becut off.

As the dielectric material of capacitors Cl and C2 begins todeteriorate, RF signals will be generated and appear at terminals l8 and19. The lower limit of the signals will lie in the range 50 to 200 KHz.Signals, whose frequencies exceed the upper limit of attenuation offilter 20, will appear across secondary 24 of transformer 21. As thevoltage across secondary 24 rises, control current will begin to flowthrough resistor R1 into node 27 of the power supply. When the level ofthe control signal exceeds a predetermined value, sufficient controlcurrent will flow to cause the voltage at node 28 to rise to about 0.7volt with respect to the emitter of transistor Q1 thus forwardly biasingthe base-emitter junction of this transistor causing it to change fromits non-conducting state to its conducting state. When transistor Q1conducts, collector current flows from terminal 29 and B+ bus 37 throughthe se-v ries resistors R3 and R4. As the voltage at node 30 betweenthese resistors drops from +20 volts to about +l8.6 volts, the biasestablished by emitter diode D3 will be overcome, and the emitter-basejunction of transistor Q2 will be forwardly biased. This transistor thusbegins to conduct as soon as transistor Q1 changes state. The flow ofcollector current in transistor Q2 through resistor R will providesufficient base current to transistor Q1 to maintain conduction of thelatter even if the rectified control signal appearing at node 28 were todecrease below the predetermined value which had been sufficient toinitially turn on transistor Q1.

From the above description, it can be seen that transistor Q1 operatesas an active device which responds to a control signal for changingstate when the level of the control signal exceeds a predeterminedvalue. Transistor Q2 corresponds to a latching means for maintaining theactive device in its changed state independently of the level of thecontrol signal, once such level exceeds the predetermined value.

When transistor Q2 conducts, collector current is furnished to resistorsR6 and R7. The voltage at node 31 between these last-mentionedresistors, due to such current, rises above the ground potential of bus35 and forwardly biases the baseemitter junction of transistor Q3 whichconstitutes a portion of the operable indicator means 14. Whentransistor Q3 conducts, its collector current will flow through theparallel combination of indicator lamp 32, damper diode D4 and coil 33of relay R1. The illumination of lamp 32 serves as a visual indicationthat a potentially dangerous condition exists with respect to thecapacitors of the tank circuit. The flow of current through coil 33 ofrelay Kl opens normally closed switch S1 disabling control means 17 andremoving power from the tank circuit.

Removal of power from the tank circuit as a consequence of the operationof means 14 will result in the disappearance of the rectified controlsignal at node 28. However, both transistors Q1 and Q2 will remain intheir conducting states because of the latching operation previouslydescribed. To reset the apparatus, switch S2 is manually closedmomentarily to apply a negative signal to terminal 34. The flow ofcollector current through resistor R5 and through series resistors R6and R7 of the reset circuit drops the voltage at node 28 to a levelbelow the ground potential of bus 35 with the result that thebase-emitter junction of transistor Q1 becomes reversed biased andconduction of this transistor ceases. When transistor 01 stopsconducting, transistor Q2 also stops conducting, thereby cutting offbase current to transistor Q3. When this last-mentioned transistor stopsconducting, relay coil 33 in its collector circuit is de-energized, thusreturning switch S1 to its closed position shown in the drawing. DiodeD4 shunting the coil 33 serves to dampen the inductive kickback producedwhen coil 33 is de-energized. Diode D5 in circuit 15 is provided toisolate the second circuit means from other circuits connected to node34.

Capacitor C5 connected between the ground bus 35 and node 36 betweendiode D5 and resistor R6, serves to smooth out transients generated inthe bias circuitry as a result of the opening and/or closing of thecontacts of reset switch S2. In the absence of capacitor C5, it would bepossible for transistor 01 to remain conducting when the reset switch S2is operated.

Upon such operation, the contacts of switch S2 are bridged causingcapacitor C5 to charge rapidly through relatively small resistor R6until the voltage at node 36 is approximately the volts at node 27. Whenreset switch S2 is released, capacitor C5 discharges through the seriescombination of the primary 24 of transformer 21, diode D1 and resistorsR1 and R7, and diode D5. The time-constant associated with thiscombination of components is much larger than the charging time-constantwith the result that node 28 is held at a negative associated with theoperation of reset switch S2.

While the above descnption 1S directed to a 3 K11: tank C11- cuit usedin an induction furnace, the invention is also applicable to statictesting of capacitors, and to tank circuits whose resonant frequency maybe several orders of magnitude higher than 3 Kill. In the latter case,the filter associated with the tank circuit would be designed to passonly those frequencies greater than the operating frequency. Thepredetermined level of the control signal necessary to cause operationof the indicator means is determined by trial and error, being dependenton the size and type of capacitor being monitored.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification as indicating the scope of theinvention.

What is claimed is:

l. A method for detecting incipient deterioration of the capacitor in anA.C. power circuit including the steps of:

a. monitoring said capacitor to detect high-frequency signals producedby said capacitor; and

b. producing an indication when the level of said signals, whose lowerfrequency limit is in the range from 50 to 200 KHz, exceeds apredetermined value.

2. Sensing apparatus for detecting incipient deterioration of acapacitor in an A.C. power circuit including:

a. first circuit means to detect the presence of signals of a frequencyin excess of a predetermined value across said capacitor;

b. operable indicator means; and

c. second circuit means responsive to said signals for operating saidindicator means when the level of said signals exceed a predeterminedvalue.

3. Sensing apparatus according to claim 2 wherein the predeterminedvalue of the frequency of said signals is in the range of 50 to 200 KHz.

4. Sensing apparatus according to claim 3 wherein said first circuitmeans includes a high-pass filter associated with said capacitor foreffecting transmission of signals greater than the lower value of saidrange.

5. Sensing apparatus according to claim 4 wherein said first circuitmeans includes an isolation transformer whose primary is part of saidhigh-pass filter, and a detector circuit associated with the secondaryof said transformer for producing a rectified control signal whose levelis functionally related to the signals passed by said filter.

6. Sensing apparatus according to claim 5 wherein said second circuitincludes an active device responsive to said control signal for changingstate when the level of said control signal exceeds a predeterminedvalue. 6

7. Sensing apparatus according to claim 6 wherein the operation of saidindicator means is responsive to a change in state of said activedevice.

8. Sensing apparatus according to claim 7 wherein said indicator meansincludes operable control means for controlling the application of powerto said capacitor, operation of said indicator means causing operationof said control means whereby power is removed from said capacitor.

9. Sensing apparatus according to claim 6 wherein said second circuitincludes latching means for maintaining said active device in itschanged state independently of the level of said control signal oncesuch level exceeds said predetermined value.

10. Sensing apparatus according to claim 9 wherein a reset circuit isprovided for selectively returning said active device to its originalstate in the absence of a control signal whose level exceeds saidpredetermined value.

1. A method for detecting incipient deterioration of the capacitor in anA.C. power circuit including the steps of: a. monitoring said capacitorto detect high-frequency signals produced by said capacitor; and b.producing an indication when the level of said signals, whose lowerfrequency limit is in the range from 50 to 200 KHz, exceeds apredetermined value.
 2. Sensing apparatus for detecting incipientdeterioration of a capacitor in an A.C. power circuit including: a.first circuit means to detect the presence of signals of a freqUency inexcess of a predetermined value across said capacitor; b. operableindicator means; and c. second circuit means responsive to said signalsfor operating said indicator means when the level of said signals exceeda predetermined value.
 3. Sensing apparatus according to claim 2 whereinthe predetermined value of the frequency of said signals is in the rangeof 50 to 200 KHz.
 4. Sensing apparatus according to claim 3 wherein saidfirst circuit means includes a high-pass filter associated with saidcapacitor for effecting transmission of signals greater than the lowervalue of said range.
 5. Sensing apparatus according to claim 4 whereinsaid first circuit means includes an isolation transformer whose primaryis part of said high-pass filter, and a detector circuit associated withthe secondary of said transformer for producing a rectified controlsignal whose level is functionally related to the signals passed by saidfilter.
 6. Sensing apparatus according to claim 5 wherein said secondcircuit includes an active device responsive to said control signal forchanging state when the level of said control signal exceeds apredetermined value.
 7. Sensing apparatus according to claim 6 whereinthe operation of said indicator means is responsive to a change in stateof said active device.
 8. Sensing apparatus according to claim 7 whereinsaid indicator means includes operable control means for controlling theapplication of power to said capacitor, operation of said indicatormeans causing operation of said control means whereby power is removedfrom said capacitor.
 9. Sensing apparatus according to claim 6 whereinsaid second circuit includes latching means for maintaining said activedevice in its changed state independently of the level of said controlsignal once such level exceeds said predetermined value.
 10. Sensingapparatus according to claim 9 wherein a reset circuit is provided forselectively returning said active device to its original state in theabsence of a control signal whose level exceeds said predeterminedvalue.